1. Field of the Invention
The present invention relates in general to a driving circuit for an optical modulator including a cascode of electrical elements, wherein output voltage is uniformly shared between the transistors of the cascode electrical elements to increase the operating range of the output voltage.
2. Description of the Related Art
Optical transmission systems with high capacity are widely implemented. To meet requirements of high speed and long distance communication, optical transmitter employing continuous-wave (CW) laser diode operating with a high-speed optical modulator is crucial. The driving circuit of the optical modulator is typically manufactured by an integrated circuit process with high breakdown voltage to output high voltage between about 3˜10 volts, thus increasing manufacturing cost. Hence, a method for forming the high-voltage driver with a low-voltage integrated circuit process is necessary.
FIG. 1 is a circuit diagram of a conventional driving circuit. The conventional driving circuit comprises transistors 10 and 11 and a current source Im. Current source Im is directed to flow through the transistors 10 or 11 toward the output terminals Vo1 or Vo2 by the differential signals respectively input to the input terminals S1 and S2. The output voltages are output completely across the transistor 10 or 11. Thus, the output voltage value is limited by the breakdown characteristics of the transistors 10 and 11.
FIG. 2 is a circuit diagram of a conventional driving circuit comprising a cascode of transistors. The high voltage output terminal of the driving circuit comprises a plurality of transistors, wherein the second transistor 12, the third transistor 13, the fourth transistor 14 and the fifth transistor 15 comprise a cascode structure for sharing the high output voltage. The driving circuit however requires an extra source-follower to couple partial output voltage to the gate of the third transistor 13. Thus, power consumption is increased and the increment of the gate delay deteriorates the high frequency output characteristics.
To solve the disadvantages of the conventional driving circuit, the present invention provides a driving circuit for outputting high voltage.